The present invention relates to position indicating devices, particularly for indicating the position of a linear element which is inaccessible to direct visual observation. More specifically, the invention is directed to devices for indicating the position of a component, such as a control rod, in a nuclear reactor.
The operation of a nuclear reactor, and particularly control of the core activity and monitoring of the core state, is effected by various elements which are mounted on drive rods which are movable to permit the elements to penetrate the core to a selected depth. While it is essetial to continuously and accurately monitor the vertical position of each such drive rod, this cannot be accomplished directly because all of the components associated with the core are enclosed in a pressure vessel preventing direct visual observation.
Heretofore, various position indicating devices for this purpose have been proposed and utilized. One class of known devices employs an array of annular sensing coils spaced apart along a vertical path and arranged to produce output signals in response to the movement of a body, such as the upper end of a drive rod, along the path, the rod being displaceable within the cylindrical region enclosed by the coils. This body is of a material selected to effect a variation in the impedance, which includes inductive and resistive components, of a coil as the body passes into the cylindrical region of the coil. The coils are arranged in series-connected pairs and the coils of each pair can be spaced apart along the vertical path in such a manner that coils of other coil pairs are interposed between the coils of a given pair. An a.c. operating voltage is applied across each coil pair and the point of connection between the coils of the pair constitutes an output at which a position indicating signal can appear. All the coils are electrically identical so that if only one coil of a pair is penetrated by the impedance-influencing body, a detectable voltage change will appear at the associated output. If both coils are penetrated or both coils are not penetrated, the coil pair will be electrically balanced, and no voltage will appear at the output.
Each coil pair output is connected to a signal producing element, such as a differential amplifier having a reference input connected to a reference potential, so that the signal producing element will produce an output signal when the detectable voltage appears at the output of its associated coil pair.
With such an arrangement, the number of signal producing elements must be equal to the number of coil pairs, which is equal to one-half the number of incremental positions that can be sensed.
Thus, if twenty different positions must be sensed, ten signal producing elements are required. The output signals from the position-sensing elements are generally processed as respective bits of a binary code so that, in the exemplary case mentioned above, a 10-bit Gray code is established.
Since it is generally desired to process such position information digitally, it would be advantageous to convert the group of signals produced by such an arrangement to an 8-bit code and a translation from a 10-bit pattern to an 8-bit code is not always a simple task. Moreover, the number of positions to be sensed varies from one system to another so that a separate conversion scheme is required for each such system. Even within a given system, the number of positions to be sensed can vary from one movable structure to another. In the case of one proposed nuclear power system, it will be necessary to accurately detect up to 23 incremental positions.